Multi-relaxation Process Research Articles

Magnetic carbon fiber/reduced graphene oxide film for electromagnetic microwave absorption

Adding 10 wt% carbon fiber (CF) into graphene oxide (GO) dispersion were performed to fabricate CF10/reduced graphene oxide (rGO) film without the use of surfactants, binders or polymer matrices. The all-carbon CF10/rGO film possesses plausible tensile strength over one page of regular A4 printing paper and improved room-temperature ferromagnetism compared with the as-assembled rGO film, with saturation magnetization of about 0.327 emu/g and coercivity of about 100.4 Oe. The CF10/rGO composite with 50 wt% filler loading exhibits a good electromagnetic wave (EMW) absorption performance in the measured frequency range of 2–18 GHz. A minimal reflection loss reaches −34.3 dB@6.2 GHz (C-band), −35.4 dB@9.6 GHz (X-band), and −33.7 dB@13.1 GHz (Ku-band) with a thickness of 3.0 mm, 2.0 mm and 1.5 mm, respectively. The enhanced complex permittivity and permeability causing by the introduction of CF in rGO film are in favor of achieving an excellent impedance matching characteristic between the carbon materials and the air, responsible for more amount of EMW incidence; furthermore, localized electronic spin states and multi-relaxation processes arisen from the topological defects, oxygen-containing groups, and interfaces are ascribed to the enhanced microwave attenuation ability.

Robust determination of relaxation times spectra of long-time multirelaxation processes.

Long-time relaxation processes occur in numerous physical systems. They are often regarded as multirelaxation processes, which are a superposition of exponential decays with a certain distribution of relaxation times. The relaxation times spectra often convey information about the underlying physics. Extracting the spectrum of relaxation times from experimental data is, however, difficult. This is partly due to the mathematical properties of the problem and partly due to experimental limitations. In this paper, we perform the inversion of time-series relaxation data into a relaxation spectrum using the singular value decomposition accompanied by the Akaike information criterion estimator. We show that this approach does not need any a priori information on the spectral shape and that it delivers a solution that consistently approximates the best one achievable for given experimental dataset. On the contrary, we show that the solution obtained imposing an optimal fit of experimental data is often far from reconstructing well the distribution of relaxation times.

Elastic Slow Dynamics in Polycrystalline Metal Alloys

Elastic slow dynamics, consisting in a reversible softening of materials when an external strain is applied, was experimentally observed in polycrystalline metals and presents analogies with the same phenomenon more widely observed in consolidated granular media. Since the effect is extremely small in metals, precise experimental techniques are needed. Reliable measurement of relative velocity variations of the order of 10−7 is crucial to perform the analysis. In addition, the grain structure and the nature of grain boundaries in metals is very different from that in rocks or concrete. Therefore, linking relaxation elastic effects to the microstructure is needed to understand the physical origin of slow dynamics in metals. Here, interpreting the relaxation phenomenon as a multirelaxation process, we show that it is sensitive to the spatial scale at the microstructural level, up to the point of allowing the identification of the existence of features at different spatial scales, particularly distinguishing damage from microstructural inhomogeneities.

A versatile acoustic gas sensing method via extracting intrinsic molecular internal specific heat

Gas-coupled acoustic techniques show advantageous capability in analyzing gas mixtures. It is still a challenge to identify the intrinsic characteristics of gas compositions from the acoustic measurements. In this work, we propose an acoustic method to extract the intrinsic internal specific heat of gas composition from molecular relaxation for gas sensing. The internal specific heats of multi-relaxation process in a gas mixture are fast synthesized using measurements of sound speed and absorption at several fixed frequencies. The relative change of internal specific heat due to variation of environmental temperature is employed to identify gas species, and the ratio of the internal specific heat of mixture to that of corresponding pure gas determines the composition concentration. The robustness of the proposed gas sensing method is validated with multi-component mixtures. The acoustic sensing method provides an effective route to identify the intrinsic characteristics of gas compositions in unknown complex gas mixtures.

Photoelectrochemical Study of the Delafossite AgNiO2 Nanostructure: Application to Hydrogen Production

Abstract AgNiO2 is a semiconductor crystallizing in the delafossite structure; it is prepared by the hydrothermal route, and the photoelectrochemical properties are studied for the first time. The TG/DSC analyses show a low stability not exceeding 290 °C before its reduction into Ag and NiO. The direct bandgap energy of the bulk material is 0.87 eV, due to the d–d transition of Ag+ linearly coordinated. AgNiO2 is chemically stable in the pH region (4–14); a flat band potential of −0.022 VRHE with p-type behavior, inferred to oxygen insertion is reported in KOH solution (10−2 M). The holes density (1.92 × 1022 cm−3) agrees with a semi-metallic behavior. Positive potentials give rise to surface oxidation of AgNiO2 in the diffusion plateau before oxygen evolution. The electrochemical oxygen insertion, investigated by chrono-amperometry, is found to be slow with a diffusion coefficient of ∼8 × 10−16 cm2 s−1. The Nyquist plot exhibits a semicircle centered below the abscissa axis, whose diameter 4200 Ω cm2 decreases down to 760 Ω cm2 under visible illumination. Such results indicate dipolar and multi-relaxation processes and confirm the existence of the optical gap. The conduction band (−0.88 VRHE) derived from Ag+: 4d orbital is more cathodic than the potential of H2O/H2 (∼−0.64 VRHE) level and hydrogen is evolved under visible irradiation. An evolution rate of 1.43 mL g−1 min−1 at pH ∼ 12.8 is obtained with a light-to-chemical energy efficiency of 2.40%.

Synthesis and enhanced microwave absorption performance of CIP@ SiO2@Mn0.6Zn0.4Fe2O4 ferrite composites

Novel core@shell@shell structured carbonyl iron powder (CIP)@SiO2@ Mn0.6Zn0.4Fe2O4 ferrite composites having great microwave absorption characteristics were successfully synthesized using chemical co-precipitation. The microstructure, morphology and microwave absorption performance of CIP@SiO2@Mn0.6Zn0.4Fe2O4 ferrite composites were systematically investigated. The results show that both coating layers of SiO2 and Mn0.6Zn0.4Fe2O4 are uniform and fine. The prepared CIP@SiO2@Mn0.6Zn0.4Fe2O4 composites in this study display better dielectric loss and frequency characteristic at high frequency than CIP, CIP@SiO2 and CIP@Mn0.6Zn0.4Fe2O4. The results of electromagnetic parameters also indicate that the enhanced microwave absorption performance primarily originates from the substantial contributions of magnetic loss to reflection loss. The optimal value of reflection loss of CIP@SiO2@Mn0.6Zn0.4Fe2O4 having the thickness of 2 mm can reach up to −44.24 dB at 11.57 GHz as well as the bandwidth (RL ≤ −10 dB) from 9.04 to 16.16 GHz among the frequency range from 2 to 18 GHz. In addition, the greatly enhanced microwave absorption performance is attributed to good wave impedance matching, high attenuation constant, multi-relaxation processes and multiple interfacial polarization.

A simple measurement method of molecular relaxation in a gas by reconstructing acoustic velocity dispersion

Recently, a decomposition method of acoustic relaxation absorption spectra was used to capture the entire molecular multimode relaxation process of gas. In this method, the acoustic attenuation and phase velocity were measured jointly based on the relaxation absorption spectra. However, fast and accurate measurements of the acoustic attenuation remain challenging. In this paper, we present a method of capturing the molecular relaxation process by only measuring acoustic velocity, without the necessity of obtaining acoustic absorption. The method is based on the fact that the frequency-dependent velocity dispersion of a multi-relaxation process in a gas is the serial connection of the dispersions of interior single-relaxation processes. Thus, one can capture the relaxation times and relaxation strengths of N decomposed single-relaxation dispersions to reconstruct the entire multi-relaxation dispersion using the measurements of acoustic velocity at 2N + 1 frequencies. The reconstructed dispersion spectra are in good agreement with experimental data for various gases and mixtures. The simulations also demonstrate the robustness of our reconstructive method.

Capturing molecular multimode relaxation processes in excitable gases based on decomposition of acoustic relaxation spectra

Existing two-frequency reconstructive methods can only capture primary (single) molecular relaxation processes in excitable gases. In this paper, we present a reconstructive method based on the novel decomposition of frequency-dependent acoustic relaxation spectra to capture the entire molecular multimode relaxation process. This decomposition of acoustic relaxation spectra is developed from the frequency-dependent effective specific heat, indicating that a multi-relaxation process is the sum of the interior single-relaxation processes. Based on this decomposition, we can reconstruct the entire multi-relaxation process by capturing the relaxation times and relaxation strengths of N interior single-relaxation processes, using the measurements of acoustic absorption and sound speed at 2N frequencies. Experimental data for the gas mixtures CO2–N2 and CO2–O2 validate our decomposition and reconstruction approach.

Investigation of structural and electrical properties of CuO modified SnO2 nanoparticles

Nanocrystalline SnO2 was prepared by the co-precipitation method and doped with CuO. Crystallite sizes were estimated by Scherrer formula to be of the order of 90nm and lesser. A.C. conduction behaviour of the samples has been described by the different hopping processes. From impedance analysis multi-relaxation processes in the samples are evident. Shrinkage of Cole–Cole plots with increase in temperature is also observed in our case which shows role of grain and grain boundary effects in the conduction mechanism with rise in temperature. Dielectric constant decreases with frequency but increases with rise in temperature which is governed by different components of polarizability (deformational and relaxational polarization). Dielectric constant increases with rise in temperature but at higher frequency range it takes constant value. Impedance analysis is used to explain the effects of grain and grain boundary on transport mechanism of undoped and CuO modified SnO2 nanoparticles.

AC transport properties of nanocrystalline SnO 2 semiconductor

Nanocrystalline SnO 2 materials were prepared by the chemical co-precipitation route by adding ammonia solution to 0.1 M solution of SnCl 4·5H 2O. The resulting precipitate after thorough washing with distilled water and calcination at 600 °C for 10 h was investigated by XRD for phase identification and crystallite size determination. The materials have been found to be polycrystalline SnO 2, possessing tetragonal rutile crystal structure and nanocrystalline in grain size of approximately 30 nm. The TEM micrograph shows agglomerated particles (cluster of primary crystallites) with an average size of 37.4 nm. A corresponding selected area electron diffraction pattern reveals the different Debye rings of SnO 2, as analyzed in XRD. The complex dielectric constant ɛ * has been found to vary with frequency which is attributed to the multi-relaxation time constants of the energy states responsible for conduction mechanism. At any particular frequency, ɛ * has been found to increase with temperature. The frequency dependence of loss tangent tan δ has been explained with the help of the equivalent circuit model. The observed frequency dependence of ac conductivity has been found to obey the power law: σ ac ∝ ω S, where variation of S with temperature indicates multi-hopping conduction mechanism in nanocrystalline SnO 2 samples. The complex impedance plots of Z′ versus Z″ at different temperatures have been found to be single semicircular arcs with a non-zero intersection with the real axis in the high frequency region and have their centres lying below the real axis at a particular angle of depression, indicating multirelaxation processes in the material.

Anomalous Dependence on the Diffusion Coefficients of the Ionic Relaxation Time in Electrolytes

We show, by using a numerical analysis, that the dynamic toward equilibrium for an electrolytic cell subject to a step-like external electric field is a multirelaxation process when the diffusion coefficients of positive and negative ions are different. By assuming that the diffusion coefficient of positive ions is constant, we observe that the number of involved relaxation processes increases when the diffusion coefficient of the negative ions diminishes. Furthermore, two of the relaxation times depend nonmonotonically on the ratio of the diffusion coefficients. This result is unexpected, because the ionic drift velocity, by means of which the ions move to reach the equilibrium distribution, increases with increasing ionic mobility.

Frequency dependence of acoustic properties of aqueous glucose solutions in the VHF/UHF range.

The bioultrasonic spectroscopy system was employed for measurements of velocity and attenuation coefficient of glucose solutions in the VHF/UHF range. The relation between the slope of the square of velocity and the relaxation parameters, and the relation between the frequency exponent on attenuation coefficient and the relaxation parameters are investigated. In order to carry out numerical calculations, a model for a single relaxation process is employed, wherein the attenuation coefficient is expressed as (A/( 1 + (f/falpha)2) + B)f2 where falpha is the attenuation relaxation frequency, and A and B are constants. The numerical calculations show that the slope of the square of the velocity is determined uniquely by the velocity relaxation frequency fv and v(infinity)2 - v(0)2 where v0 is the zero-frequency velocity and v(infinity) is the infinite-frequency velocity, and that the frequency exponent on the attenuation coefficient is determined uniquely by falpha and A/B. For experimental considerations, the velocities and the attenuation coefficients of 5, 15, and 25% concentration aqueous solutions of glucose were measured in the frequency range 20 to 700 MHz. The data for the 5 and 15% aqueous solutions can be explained using the single relaxation model. However, the data for the 25% aqueous solution suggest the existence of multirelaxation processes.

Nanostructural Fluctuation in Radiation-Amorphized Alloys

ABSTRACTSpatio-temporal nanostructural fluctuations brought about by transient, metastable atom-cluster formation and the manifold nature of inherent atomic ordering in electron-irradiation-amorphized NiTi were investigated by using a combination of in situ observations inside a high-resolution high-voltage electron microscope (HR-HVEM) and image analyses of molecular-dynamics-simulated atom configurations. Nanometer-sized clusters were found to appear and disappear in the irradiated region. The random formation and annihilation of such nanoclusters are believed to be responsible for nanostructural fluctuations which appear to be related to transitions among manifold inherent structural states, involving multirelaxation processes. Temporal fluctuations in the amorphized structure were manifested through the dose-dependent local amorphization parameter, potential energy, volume, and inherent cluster bonding. The observed fluctuations obey a universal power law. Within the framework of the multi-Lorentzian picture, the resultant power law describes the distribution of multirelaxation times or cluster lifetimes. In addition, a unified relation for the temporal autocorrelation function for such fluctuation phenomena has been determined.

A NEW TECHNIQUE FOR OBTAINING THE “MOBILITIES SPECTRUM” TRANSIENT THERMOELECTRIC EFFECT

We introduced a new technique called “Transient Thermoelectric Effect (TTE)”, which was based on carrier-diffusion as irradiated by a pulsed laser. The observed TTE voltages decayed exponentially with time, showing a multi-relaxation process with characteristic relaxation times τi(i=1,2,3,…) for thermal diffusions of photo-induced conduction carriers, analyses of these results could give valuable information about carrier mobilities, effective masses and Fermi surfaces. The principle and the main analysis method were also described in detail.

Valence-band changes in Sb2-xInxTe3 and Sb2Te3-ySey by transport and Shubnikov-de Haas effect measurements.

Measurements of galvanomagnetic effects in the temperature range 4.2--300 K and photoinduced ``transient thermoelectric effect'' (TTE) along the ${\mathit{C}}_{2}$ axis at 300 K have been made for two types of solid solutions of semiconductors ${\mathrm{Sb}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$${\mathrm{Te}}_{3}$ (0\ensuremath{\le}x\ensuremath{\le}0.4) and ${\mathrm{Sb}}_{2}$${\mathrm{Te}}_{3\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Se}}_{\mathit{y}}$ (0\ensuremath{\le}y\ensuremath{\le}1.8). By incorporating In atoms into the ${\mathrm{Sb}}_{2}$${\mathrm{Te}}_{3}$ lattices, Hall coefficients, Hall mobilities, and the frequencies of Shubnikov--de Haas (SdH) oscillations are varied systematically. For ${\mathrm{Sb}}_{2}$${\mathrm{Te}}_{3\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Se}}_{\mathit{y}}$, the Hall mobility is decreased with y up to y=0.7 and then increased appreciably in the range 0.7y1.8, and a frequency component of SdH oscillations is observed for y\ensuremath{\ge}0.25. The observed TTE voltages decay exponentially with time, showing a multirelaxation process with characteristic relaxation times ${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}$ (i=1,2, . . .) for thermal diffusions of photoinduced conduction carriers, whose analyses give valuable information about carrier mobilities and effective masses. In the host material ${\mathrm{Sb}}_{2}$${\mathrm{Te}}_{3}$, four relaxation times ${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}$ (i=1--4) are found, which are attributable to holes in the anisotropic upper and lower valence bands with effective-mass anisotropies of about 3. In addition, we have found two kinds of extra relaxation times ${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}$ (i=5 and 6) for y>0.6 in ${\mathrm{Sb}}_{2}$${\mathrm{Te}}_{3\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Se}}_{\mathit{y}}$, confirming the existence of a valence band, whose anisotropy in the effective mass along the ${\mathit{C}}_{2}$ direction is evaluated to be of the order of 2--2.5. Based on these experimental data we have proposed the most probable band model for these solid solutions.

Structural relaxation in thermorheologically complex materials

Further development and testing of the thermorheologically complex (TC) model is reported. This five-parameter models deals with structural relaxation as a multi-relaxation process. A set of relaxation mechanisms is associated with the distributions of energy barriers and Kauzmann temperatures. The energy barriers decrease and the Kauzmann temperatures increase from slow to fast mechanisms. The TC model can be fully defined using equilibrium data in which case it has only three adjustable and two dependent parameters. The TC model successfully accounts for structural relaxation of glass plates quenched into molten salt and structural relaxation of rapidly cooled fibers — the experimental data that defined the Tool-Narayanaswamy model.

A new graphical representation for dielectric data

A method of graphically representing complex dielectric data is described, which is particularly useful for situations where the Cole–Cole representation is unsuitable. The representation emphasizes the orientational dipolar contribution to the conductivity by plotting σχ″=ωεv(ε′−ε∞) vs σχ′=ωεvε″. We discuss the utility of the representation for conducting dielectrics and for studying multirelaxation processes, particularly modes with smaller amplitudes (absorption) at high frequencies. For a Debye relaxation with dc conductivity, this representation leads to a semicircle, whereas the Cole–Cole plot shows a divergence. Even in the absence of conductivity, this representation is useful particularly at high frequencies, where it enables the identification of multirelaxation processes not apparent in the Cole–Cole representation. A particularly striking example is the clear observation of two relaxation processes in dielectric data on 1-propanol. We also show that in this σχ representation, all the well-known functional forms such as Cole–Cole and Cole–Davidson, approach an asymptotic slope at high frequencies. Applications of the σχ representation to analysis of dielectric spectra taken up to 20 GHz are discussed for pure glycerol and LiCl/propanol solutions.

Separation of NMR Spin-Spin Multi-relaxation Process and Imaging – Phantom Investigation

A physical-mathematical model was proposed for the NMR relaxation processes, which can automatically identify the multi-relaxation process; the automatic separation of the spin-spin relaxation processes of water sample in a home made phantom was carried out; two kinds of proton pure spin-spin relaxation time images and two kinds of proton computed pure density images have been obtained on a 1.0 T supercondutive magnet MR-Scanner.

An acoustic study of the mixed-alkali-mixed phase β-aluminas. Part 1: Naβ″/βAl 2O 3, Kβ″/βAl 2O 3 and (Na 0.6K 0.4)β″/βAl 2O 3

The relative attenuation of compressional sound waves of frequencies 10–60 MHz in mixed alkali (Na/K) mixed phase (β″/β)-aluminas is reported for temperatures 80–550 K. The internal friction peaks shift to higher frequencies at higher temperatures and are attributed to Na + interactions in Naβ″/β alumina and Na + and K + in NaK β″/β alumina. The broad attenuation peaks occuring at low temperatures (< 300 K) and at higher temperatures (> 400 K) suggest multi-relaxation processes giving a distribution of activation energies. The estimated average activation energy for Na + diffusion in Naβ″/βAl 2 O 3 at low temperatures and high temperatures is 0.183 eV and 0.387 eV respectively. In the NaK β″/βAl 2o 3 samples, the Na + values were 0.239 eV and 0.386 eV, respectively. The estimated average activation energies for K + diffusion at low and high temperatures in the Kβ″/β-alumina samples were 0.269 eV and 0.371 eV and for K + in the NaK β″/β samples, 0252 eV and 0.339 eV, respectively. The low temperature attenuation peaks were interpreted in terms of ionic interaction in the bulk and the high temperature peaks were related to interactions in the grain boundaries. The measured activation energies confirmed these interpretations. A reversal of the temperature appearance of the Na + and K + high temperature peaks in the NaKβ″/βAl 2 O 3 is explained by the disorder at the grain boundaries.

Kinetics of activation and desensitization in receptor proteins.

AbstractReceptors are functional membrane proteins on the cell surface that recognize external signals and trigger biological responses by generating intracellular signals. Due to prolonged exposure to external signals, receptors are often desensitized and no longer produce intracellular signals. This simple control mechanism may work without negative‐feedback regulation from another molecule if the active state of a receptor reflects a transient metastable molecular structure. A theoretical framework is developed to identify a metastable state associated with a conformational transition of protein molecules, in which a transient state can be observed somewhat above the equilibrium transition point. The conducting state of the acetylcholine receptor may thus represent a metastable state associated with a conformational transition from the resting state to the desensitized state. Similarly, the conducting state of the voltage‐sensitive sodium channel may represent a metastable state associated with a conformational transition from the resting state to the refractory state. The rates of appearance and disappearance of the transient state, as well as the equilibrium ratio of the two preexisting states, can be estimated from the free energy of protein structure. The appearance of the transient state is generally a multirelaxation process and may show a time lag, while the disappearance is a slower single‐relaxation process.